Characterization of an RF-driven argon plasma at atmospheric pressure using broadband absorption and optical emission spectroscopy

Abstract

Atmospheric pressure plasmas in argon are of particular interest due to the production of highly excited and reactive species enabling numerous plasma-aided applications. In this contribution, we report on absolute optical emission and absorption spectroscopy of a radio frequency (RF) driven capacitively coupled argon glow discharge operated in a parallel-plate configuration. This enabled the study of all key parameters including electron density and temperature, gas temperature, and absolute densities of atoms in highly electronically excited states. The space and time-averaged electron density and temperature were determined from the measurement of the absolute intensity of the electron-atom bremsstrahlung in the visible range. Considering the non-Maxwellian electron energy distribution function, an electron temperature (T e ) of 2.1 eV and an electron density (n e ) of 1.1 × 10 19 m −3 were obtained. The time-averaged and spatially resolved absolute densities of atoms in the metastable (1s 5 and 1s 3 ) and resonant (1s 4 and 1s 2 ) states of argon in pure Ar and Ar/He mixture were obtained by broadband absorption spectroscopy. The 1s 5 metastable atoms had the largest density near the sheath region with a maximum value of 8 × 10 17 m −3 , while all other 1s states had densities of at most 2 × 10 17 m −3 . The dominant production and loss mechanisms of these atoms were discussed, in particular, the role of radiation trapping. We conclude with a comparison of the plasma properties of the argon RF glow discharges with the more common He equivalent and highlight their differences.

Plasma-droplet interaction study to assess transport limitations and the role of ⋅OH, O⋅,H⋅,O2(a 1Δg),O3, He(23 S) and Ar(1s 5) in formate decomposition

“…The uncertainty in n e due to the addition of H 2 continuum to the overall fit does not necessarily increase, however, it impacts the uncertainty in T e more significantly. The electron properties (n e and T e ) determined from the continuum emission were compared with power balance estimates previously [49], and while for helium, a good agreement was obtained, in the case of argon, significant discrepancies were found which might suggest that the n e and T e determined for the He/Ar/H 2 O mixture might be less accurate and further validation of the continuum emission approach might be beneficial. The corresponding densities near the sheath edge are shown in figure 7.…”

“…Ar metastable (1s 5 ) density The absolute densities of atoms in the metastable state of argon (1s) in He/Ar/H 2 O plasma were measured using BAS. The optical setup for the spectrally resolved measurements is identical as described in [49]. The time-averaged densities were measured across the plasma gap with a spatial resolution of 40 µm without dispensing the droplets.…”

“…where d and V P are the calibration distance and the plasma volume, respectively. The details of the measurement technique and data analysis can be found in [49]. For the electron-atom bremsstrahlung cross-section of gas mixtures (σ B ea ), the elastic electronatom momentum transfer cross-sections were obtained from the Phelps database for He, Ar and O 2 [65][66][67], and the Itikawa and Hayashi databases for H 2 O [68][69][70][71].…”

“…When using a Maxwellian EEDF, T e is smaller up to 35% and n e is larger up to 112%. A more detailed description on the effect of the EEDF on T e and n e determination can be found in [49]. For plasmas containing H 2 O, the H 2 continuum also significantly contributes to the emission spectrum besides the neutral bremsstrahlung radiation [63].…”

“…In our previous work, an electron density of ∼ 10 19 m −3 was determined in Ar plasma (13.5 W) [49]. However, in this study, in a mixture of He/Ar/H 2 O plasma, n e was estimated to be around 1.5 × 10 17 m −3 , which is more similar to a helium plasma [49]. This is consistent with the dilution of Ar with 83% helium and the presence of H 2 O vapor, an electronegative gas.…”